Chapter 7 - X-ray diffraction tomography: Application to imaging heterogeneous systems

Abstract

This chapter describes a relatively new tomographic technique, where the imaging process is based upon the measurement of diffracted x-radiation, rather than the attenuation of an x-ray beam. By recording the angular distribution of scattered photons as an object is scanned with a pencil beam of radiation, tomographic data sets are produced that contain information about the scattering properties of the materials within the object. The tomographic data sets are then processed to produce images, which map the spatial distribution of these scattering properties throughout the interior of the object. The reconstructed scattering information at a given spatial coordinate may then be used as a means for the characterisation or identification of the material at that site. The use of scattered radiation as a basis for tomographic imaging is particularly effective when the constituents within the object preferentially scatter into different, well-defined angular ranges. This may occur when the radiation is diffracted by materials having an ordered arrangement of scattering sites. Even when the arrangement of scattering sites is of a statistical nature, as in liquids and amorphous materials, the height and angular position of the associated diffuse x-ray diffraction peaks, may be used as a basis for materials discrimination.Fixed-energy x-ray transmission computed tomography produces images with a single value at each spatial image coordinate, or pixel, and this value represents the x-ray linear attenuation coefficient, μ. The angular variation in the radiation scattered from a material may also be used as a basis for materials discrimination.